1. Field of the Invention
The present invention relates generally to imaging measurement systems. More particularly, the invention relates to precise calibration of electrostatic deflection systems used, for example, in microscopes that employ scanning beams.
Further, the present invention relates to a position controlled wafer stage for improving and maintaining precise calibration of the deflection system.
2. State of the Art
It is known to use electromagnetic systems in microscopes such as scanning electron microscopes (SEMS) for measurement and inspection purposes. Scanning electron microscopes are presently replacing traditional optical microscopes for microelectronics inspection and metrology of applications in semiconductor manufacturing. Metrology tools are, for example, used to measure patterns (e.g., critical dimensions) formed on semiconductor wafers during fabrication.
The short wavelengths of scanning electron microscopes have several advantages over conventionally used optical microscopes. For example, scanning electron microscopes can achieve resolutions from about 100 to 200 Angstroms, while the limiting resolution of optical microscopes is typically about 2,500 Angstroms. Further, scanning electron microscopes provide depths of field several orders of magnitude greater than optical microscopes. Despite the accuracy and precision of present scanning electron microscopes, enhanced instrument specifications and capabilities are required as parameters (e.g., critical dimensions) to be inspected in the submicrometer ranges.
An article entitled "Microelectronics Dimensional Metrology in the Scanning Electron Microscope", Parts I and II, Solid State Technology by Michael T. Postek et al. (November 1986), describes a typical SEM wafer inspection instrument. As described therein, a focused electron beam is scanned from point to point on a specimen surface in a rectangular raster pattern. Accelerating voltage, beam current and spot diameter are optimized for the specific application and specimen composition.
As the scanning electron beam contacts the surface of a specimen, backscattered and/or secondary electrons are emitted from the specimen surface. Semiconductor inspection, analysis and metrology is performed by detecting these backscattered and/or secondary electrons. A point by point visual representation of the specimen is obtained on a CRT screen as the electron beam controllably scans the specimen.
Although known scanning electron microscopes are able to provide a resolution adequate for semiconductor manufacturing, several factors limit their resolution. For example, electron beam deflection errors can be introduced into the raster scan of the electron beam.
Wafer positioning stages presently used in the metrology field are movable in two perpendicular axes. These two axes form a plane which is approximately perpendicular to a directed field of view over a wafer surface located on the stage.
A separate glass scale is associated with each of the x and y axes, respectively. Each glass scale enables movement of the wafer stage to be determined with micron resolution. However, this two-axis stage position control produces an inherent ambiguity between rotation and displacement of the stage or wafer surface relative to the stage.
More particularly, rotation of the stage relative to the two scales can affect the detection in the two axes of measurement. Such rotation will thus inhibit accurate imaging of a wafer surface.
Accordingly, it would be desirable to provide a microscope with improved imaging resolution. Such resolution would permit the manufacture of VLSI devices with enhanced precision. Further, it would be desirable to permit accurate calibration despite rotation of the wafer positioning stage.